1. Field of the Invention
The present invention relates to an apparatus for operating a fluorescent lamp which is used as a light source in a copying machine or facsimile apparatus and which exposes an original document to light.
2. Description of Related Art
Hitherto, a light source for exposing an original document to light in a copying machine, such as an electrophotographic apparatus, has mainly been a halogen lamp or a fluorescent lamp. A fluorescent lamp is usually employed in a low-speed machine that does not require a large amount of light. Since the fluorescent lamp consumes a very small amount of electric power, employment of a fluorescent lamp in higher speed machines is preferable.
An example of a drive circuit and of a control circuit for a fluorescent lamp to be mounted on a copying machine are shown in FIG. 17.
Reference numeral 1 represents a fluorescent lamp, 2 represents an inductor for limiting an electric current for turning on the fluorescent lamp 1, 3 represents an inverter transformer, 4 represents an inverter transformer drive transistor, 5 represents a resonance capacitor, 6 represents a DC stabilizing power source in the apparatus, 7 represents a diode bridge, 8 represents a control transistor for turning the fluorescent lamp 1 on and off, 9 and 10 represent bypass capacitors, 11 and 12 represent filaments in the fluorescent lamp 1, and 13 represents an oscillator. Reference numeral 20 represents a commercial AC power source, 21 represents an AC-to-DC converter for generating a low-level DC output voltage VDD for use in the apparatus from the commercial AC power source 20, 22 represents a system controller for controlling the system of the apparatus, and 23 represents a mode signal for controlling the output voltage VDD from the AC-to-DC converter 21.
The operation of the circuit shown in FIG. 17 will now be described with reference to a timing chart shown in FIG. 18. When electric power is supplied to the apparatus, the apparatus is brought into a standby mode. At this time, a standby mode signal is supplied from the controller 22 to the AC-to-DC converter 21. This results in an output voltage VDD from the AC-to-DC converter 21 that is lower than the voltage level required in the copying operation, the lower voltage level being 12 V in this embodiment.
The fluorescent lamp 1 is turned off during the standby mode. However, a low level pre-heating electric current must be supplied to the filaments 11 and 12 in order to smoothly turn on the fluorescent lamp 1 when the copying operation starts. In this case, the output voltage from the AC-to-DC converter 21 is applied to an end of the input of the inverter transformer 3. Another end of the input is switched on and off at a frequency of 30 KHz in accordance with an output from the oscillator 13 input to the base of inverter transformer drive transistor 4. Thus, a high level voltage Vh resulting from the winding ratio is outputted from the inverter transformer 3. Since the Light On signal is off, the diode bridge 7 is short-circuited by the control transistor 8 at this time, and the output from the inverter transformer 3 forms a series circuit with the inductor 2, the filament 11 in parallel with the bypass capacitor 9, and the filament 12 in parallel with the bypass capacitor 10. Therefore, in the case where the impedance of the filament is quite low, the output coil electric current is approximately as follows: EQU Vh/j.omega.L.
A value obtained by subtracting an electric current flowing through the bypass capacitors 9 and 10 from the foregoing value is the pre-heating electric current which flows through the filaments 11 and 12 of the fluorescent lamp 1. In this embodiment, a filament electric current of 350 mA flows at this time; this state is called a half pre-heating state.
When a copying key is then depressed, a copy mode signal is supplied from the controller 22 to the AC-to-DC converter 21. As a result, the output voltage V.sub.DD from the AC-to-DC converter 21 is raised to a level with which each unit in the apparatus can be operated, the raised level being 24 V in this embodiment. Thus, the output coil voltage from the inverter transformer 3 is substantially doubled, and therefore the pre-heating electric current is substantially doubled. In this embodiment, a filament electric current of 700 mA flows at this time and the foregoing state is called a full pre-heating state. The full pre-heating state that exists immediately before the fluorescent lamp is turned on is effective to quickly turn on the fluorescent lamp 1. Usually, the foregoing process is a required sequence.
The short-circuit state of the diode bridge 7 is suspended when the original document is exposed to light by turning on the Light On signal which turns on the control transistor 8. This causes the output voltage from the inverter transformer 3 to be applied to both ends of the fluorescent lamp 1. As a result, the fluorescent lamp 1 is turned on. When the exposure time expires, the diode bridge 7 is again short-circuited, causing the fluorescent lamp 1 to be turned off. The full pre-heating mode continues until the apparatus completes the post-exposure processing of the copying operation and the apparatus then returns to the standby mode.
A high-speed machine is able to employ a fluorescent lamp as the exposing light source only if the fluorescent lamp is capable of emitting a large amount of light and has a long operating life. For the fluorescent lamp to emit a large amount of light, to have stable light adjustment capability for preventing flickers, to lengthen the life of the filaments thereof, and to prevent blacking of the tube surface near the filaments, the temperature of each filament at the time of the light adjustment must be controlled precisely.
However, the conventional method cannot, in the operation sequence of the apparatus, precisely control the pre-heating electric power for use in the light adjustment process. Use of the conventional method also causes exhaustion of electron emission substances in the filaments, disconnection of the filaments and reduction in the light quantity due to excess blacking of the tube surface, thereby shortening the operating life of the filament.